She knows advanced magic, and everyone knows she knows advanced magic because she's a Crimson Demon. She posted a note on the adventurer board at some point, but for some reason the only people she attracts aren't looking for a qualified mage, even though Crimson Demons are in apparently high demand.
I feel like Yunyun because I'm about to answer my own question.
There are large light novel spoilers forthcoming. You have been warned.
She can't make friends or join a party because she's an idiot.
As revealed in the Mask spinoff, Yunyun frequently goes on high level solo quests. In addition, when other adventurers get in trouble, Yunyun frequently rescues them. Every time the party tries to thank her, she says "Sorry for doing something unnecessary" and runs away.
Because of this and the fact that she's always solo, and extremely powerful, rumors have spread through the guild that she hates associating with people. It is believed by the general population that she is a powerful loner player, like Kirito from SAO basically.
Added to the fact that she is good looking, most of the beginner adventurers in the town believe that she is too good to be friends or party members with them, because she is a beautiful and powerful mage who doesn't like to associate with others.
Yunyun is indeed a very capable magician that would be far more useful in any party compared to Megumin. Her inability to find a party is instead, essentially because of she is a shy chuunibyou that doesn't fit in with anyone and struggles with the social aspect of finding a party rather than the physical capabilities. Also, she has a very different personality to the intense demeanour the crimson clan is associated with so I guess the people who would seek a crimson magician are not the sort to ask her. Possibly could be a little bit of bad luck too, I don't think her poster has been up for that long. Lastly, and probably the most important reason... its funny :D